The present invention relates to improvement of an electric power steering apparatus for assisting steering force by rotationally driving a motor for assisting steering force. Pulse with modulation (PWM) control of the motor is used with a target value of the motor current determined on the basis of the detected amount of the steering torque used as a target amount for automatic control.
In an electric power steering apparatus for rotationally driving a motor for assisting steering force by PWM control based on the current target value of the motor determined on the basis of the detected amount of steering torque and the detected amount of the driving current for the motor, steering wheel return current is fed to the motor to carry out steering wheel return control. When the steering wheel returns near its neutral position (steering angle) at which the vehicle is advanced straight ahead, the steering wheel return current is set to zero.
However, even when the steering wheel returns to its neutral position, owing to the inertia force of the motor, the steering wheel does not instantly stop rotating but goes beyond the neutral position (0 deg) and then turns back like a pendulum, until it converges on the neutral position. Because of this, it takes time before the steering wheel converges on the neutral position and stops, during which time the vehicle running condition is not stabilized. The effect of this phenomenon is especially large at high speeds.
In order to solve such problem, various proposals have been made such as one described in U.S. Pat. No. 4,754,829 which is designed to brake the motor for steering assistance when certain conditions of steering torque and steering rotation speed are satisfied, either one or both of which are changed by the vehicle speed. A steering system described in U.S. Pat. No. 4,802,544 is designed to effect braking by short-circuiting the two terminals of the motor for assisting steering force when the vehicle speed exceeds a predetermined value and steering is stopped. The steering system described in U.S. Pat. No. 4,735,271 is designed to brake the motor for assisting steering force when the steering torque does not exceed a predetermined value, steering rotation speed is more than a predetermined value, and the steering system is at the neutral position.
Also, there has been proposed a steering system as described in U.S. Pat. No. 4,735,271 which is designed to brake the motor for assisting steering force when the steering torque does not exceed a predetermined value and the rotation speed of the motor for assisting steering force is higher than a predetermined value. In all those disclosures, however, due to the abrupt exertion of the braking force, the shock of braking is conveyed to the steering wheel to give unsatisfactory braking feeling.